At the moment, methinks that major heroes (high command/leaders of factions, eg. s-t-n, reynolds, kratas, roach, me, bob etc. depending on timescale) will be heroes which spawn as your recurring ones, while lower ranking characters would be buildable but not respawn upon death, but would be re-buildable.

This also depends on their significance in the FW timeline as well as their rank and power, but these things have a rough trend of staying together.